CSS2 Attribute Selectors with Regex

Question

CSS Attribute selectors allow the selection of elements based on attribute values. Unfortunately, I've not used them in years (mainly because they're not supported by all modern browsers). However, I remember distinctly that I was able to use them to adorn all external links with an icon, by using a code similar to the following:

a[href=http] {
    background: url(external-uri);
    padding-left: 12px;
}

The above code doesn't work. My question is: How does it work? How do I select all <a> tags whose href attribute starts with "http"? The official CSS spec (linked above) doesn't even mention that this is possible. But I do remember doing this.

(Note: The obvious solution would be to use class attributes for distinction. I want to avoid this because I have little influence of the way the HTML code is built. All I can edit is the CSS code.)

Answer 1

As for CSS 2.1, see http://www.w3.org/TR/CSS21/selector.html#attribute-selectors

Executive summary:

    Attribute selectors may match in four ways:

    [att]
    Match when the element sets the "att" attribute, whatever the value of the attribute.
    [att=val]
    Match when the element's "att" attribute value is exactly "val".
    [att~=val]
    Match when the element's "att" attribute value is a space-separated list of
    "words", one of which is exactly "val". If this selector is used, the words in the 
    value must not contain spaces (since they are separated by spaces).
    [att|=val]
    Match when the element's "att" attribute value is a hyphen-separated list of
    "words", beginning with "val". The match always starts at the beginning of the
    attribute value. This is primarily intended to allow language subcode matches
    (e.g., the "lang" attribute in HTML) as described in RFC 3066 ([RFC3066]).

CSS3 also defines a list of selectors, but the compatibility varies hugely.

There's also a nifty test suite that that shows which selectors work in your browser.

As for your example,

a[href^=http]
{
    background: url(external-uri);
    padding-left: 12px;
}

should do the trick. Unfortunately, it is not supported by IE.

Answer 2

Antti's answer is sufficient for selecting anchor's whose href's begin with http and gives a perfect rundown on the available CSS2 regex-esque attribute selectors, like so:

Attribute selectors may match in four ways:

[att]
Match when the element sets the "att" attribute, whatever the value of the attribute.
[att=val]
Match when the element's "att" attribute value is exactly "val".
[att~=val]
Match when the element's "att" attribute value is a space-separated list of
"words", one of which is exactly "val". If this selector is used, the words in the 
value must not contain spaces (since they are separated by spaces).
[att|=val]
Match when the element's "att" attribute value is a hyphen-separated list of
"words", beginning with "val". The match always starts at the beginning of the
attribute value. This is primarily intended to allow language subcode matches
(e.g., the "lang" attribute in HTML) as described in RFC 3066 ([RFC3066]).

However, here is the appropriate, UPDATED way to select all outgoing links using the new CSS3 :not pseudo class selector as well as the new *= substring syntax to make sure it disregards any internal links that may still begin with http:

a[href^=http]:not([href*="yourdomain.com"])
{
    background: url(external-uri);
    padding-left: 12px;
}

*Note that this is unsupported by IE, up to at least IE8. Thanks, IE, you're the best :P

Answer 3

Note that, in Antti's example you'd probably want to add a catch for any absolute links you may have to your own domain, which you probably don't want to flag as 'external', e.g.:

a[href^="http://your.domain.com"]
{
    background: none;
    padding: 0;
}

And you'd want this after the previous declaration.

You might also want to include the full protocol prefix, just in case you have a local document named "http-info.html" that you wish to link to, e.g.:

a[href^="http://"]
{
    background: url(external-uri);
    padding-left: 12px;
}

Note that, in both these slightly-more complex cases, you should quote the value. These work, for me, in IE7.